1. Field of the Invention
The present invention relates generally to a broadband wireless access communication system, and in particular, to a method, apparatus and system for controlling automatic retransmission request (ARQ)-related timers in a broadband wireless access communication system during a handover situation.
2. Description of the Related Art
In addition to the rapid progress in the level of performance of mobile subscriber stations and the rapid increase in a data rate of communication networks, the use of a relatively high-speed data communication function is being remarkably emphasized. In particular, receiving and reproduction of multimedia, or to permit watching of terrestrial television broadcasting programs using the mobile subscriber stations, requires a very fast data service rate.
This type of data communication is achieved by controlling data flow between a mobile subscriber station and a communication network, wherein the mobile subscriber station sends an acknowledgement signal ACK in response to received packet data and defines a time limit (timeout) for waiting for a return acknowledgement signal ACK. In particular, as a data communication rate of the mobile subscriber station is increased, to confirm stable data transmission, a sender should receive the acknowledgement signal ACK from a recipient and uses the timeout period to determine the time for waiting for the acknowledgement signal ACK. Thus, if the acknowledgement signal ACK does not exist within the determined timeout period, the sender generally performs retransmission. This technique is called an automatic retransmission request (ARQ) scheme.
As described above, as a data transmission method between a mobile subscriber station and a network, the mobile subscriber station basically sends the acknowledgement signal ACK as soon as receiving single data, and for most wireless data communications, since a delay time is relatively long due to system characteristics, the acknowledgement signal ACK should be transmitted to a sender every time data is received.
Thus, when joining a communication network managed by a base station, each mobile subscriber station performs an initial registration process, establishes a plurality of connections, and determines ARQ-related timers to be used for each connection when opening the connection. In the ARQ scheme, fixed retransmission timer values are always used, and if the fixed retransmission timers are set to a small value, there is a limit to guarantee reliability, and if the fixed retransmission timers are set to a large value, a delay and a delay variation largely increase.
The ARQ-related timers will be described in detail with reference to the 6.3.4 ARQ mechanism of the Institute of Electrical and Electronics Engineers (IEEE) 802.16d specification. However, the ARQ-related timers operate only in a normal state, and there is a problem in that a handover situation is not considered in use. Thus, if the handover situation occurs in a mobile subscriber station, a case where the mobile subscriber station waits to receive an acknowledgement signal for previously transmitted packet data may occur. Likewise, if a state of receiving data from a base station is bad, or if feedback for packet data transmitted to an upstream link cannot be received, an ARQ window does not move anymore (i.e., the window cannot be time-shifted) and an ARQ synchronization (sync) loss timeout period, which is a timer related to ARQ sync matched between the mobile subscriber station and the base station, expires, thereby resetting the ARQ timer.
To prevent this situation, if the ARQ sync loss timeout period is set for a long period of time considering handover latency, in a normal state, there is a problem that if sync loss between a sender and a recipient occurs, it will be discovered late. On the contrary, if the ARQ sync loss timeout period of the ARQ-related timers is set too short without considering the handover latency, resetting of the ARQ timer occurs too frequently.
In a state where the handover situation is maintained, besides the ARQ sync loss timeout period, if an ARQ retry timeout period expires, even if the recipient has not received packet data, the sender will retransmit the packet data. Accordingly, if the ARQ retry timeout period is set for a long time period considering the handover latency, retransmission begins late in a case where feedback does not return in a normal state, and on the contrary, if the ARQ retry timeout period is set short, retransmission frequently occurs.
In a case of ARQ block lifetime, which is a maximum time interval taken to transmit one ARQ block, of the ARQ-related timers, when the ARQ block lifetime expires, a corresponding ARQ block is discarded. If the ARQ block lifetime is set for too short a time period, the corresponding ARQ block is discarded without being sufficiently retransmitted, and on the contrary, if the ARQ block lifetime is set for too long a time period considering the handover latency, a buffer overflow will frequently occur.
As described above, since fixed ARQ-related timers are always used in the conventional ARQ scheme, a state change of a communication network, e.g., a handover situation, cannot be efficiently dealt with. Thus, in the conventional ARQ scheme, if the ARQ-related timers are set to a small value, there is a limit to guarantee reliability, and if the ARQ-related timers are set to a large value, a delay and a delay variation largely increase.